There are many chemical processes where it is necessary to bring into contact a gas and solid particulate matter, or solids, or particles. Frequently, chemical reactions as well as physical phenomena take place during such contact. In most cases, gas and solids must be in contact for a minimum time period, if the contact is for a shorter period the desired chemical or physical reaction will not take place or will be incomplete. In some cases there is a maximum contact time period, beyond which less than optimum or undesirable results will be obtained. It is highly desirable to conduct gas/solid contacting processes in a continuous or semicontinuous manner rather than as a batch operation.
A contacting zone is usually maintained at some positive pressure (above atmospheric) of the contacting gas. Particles must be introduced and withdrawn from the pressurized zone without losing contacting gas to the atmosphere. It is often necessary to maintain the internal pressure of the contacting zone at a particular level or within a certain range. Contacting zone pressure may be higher than that of the zone from which solids are provided to the contacting zone. Feeding solids into a zone against a high pressure poses numerous problems. When equipment, such as screw conveyors or star valves is used, contact between equipment and solids degrades the solids particles by breaking them into smaller particles and causes equipment wear. It is difficult to maintain effective sealing to prevent escape of gas from the contacting zone and equipment maintenance costs are high. These problems are magnified when solids or gas or both are at elevated temperatures. Pressure lock systems having catalyst control valves through which the solids pass have been used for feeding solids into a pressurized zone, but the valves are highly subject to wear and can promote attrition of the particulate material.
U.S. Pat. No. 2,851,401, cited above, discusses the problems involved in solids transfer and teaches a method of transferring solids without the use of particle control valves. Instead, the flow of particles is regulated by varying the upward gas velocity through particle transfer conduit that connect particle retention zones. Particle flow is stopped by increasing the gas flux until it holds up the particles in the conduit. U.S. Pat. No. 4,576,712 shows a specialized piping and valve system for controlling gas flow and pressures between particle retention zones.
Although these arrangements do eliminate valves that directly control particle transport, the valves that control gas flow are still eroded by fine particles that are entrained in the gas flowing out of the particle zone. Filters and traps have been used to catch the fine particles and prevent damage to the valves. However, the filters tend to become plugged and can interfere with particle transport by retarding or preventing degassing of the collection zones.